The present invention relates generally to radio frequency (RF) signal processing and, more particularly, to a uniquely configured switched multiplexer that uses a bank of filters to filter a signal generated by a multi-octave transmitter whose frequency may vary over time. The switched multiplexer is also adapted to create a stable output impedance regardless of which one of the filters among the bank is selected, across the entire frequency range (e.g., 2-18 GHz) in order to reduce the phase and amplitude error of the signal.
In the field of RF signal processing, multi-octave transmitters are used wherein it is necessary to filter off transmitter harmonics associated with the RF signal. More specifically, it is typically necessary to filter off or suppress second and third harmonics in order to prevent interference of the signal with other radio systems. In multi-octave systems, such filtering of secondary and third harmonics cannot be performed by a fixed filter. For example, a sub-octave transmitter might be configured to operate in the range from about 2-3 GHz and have a second harmonic which is in the range of from about 4-6 GHz. Including a 3 GHz low-pass filter (LPF) allows passage of frequencies that are below 3 GHz while blocking signals that are above 3 GHz. However, using a fixed LPF for a multi-octave transmitter operating in the range from about 2-18 GHz would require a rating of about 18 GHz in order to prevent obstruction of the desired signal. Unfortunately, the use of an 18 GHz filter would also allow passage of the second harmonic from the transmitter when operating in the range of from about 2-9 GHz.
Attempts to overcome the above-described problem include providing a bank of selectable filters to reduce the second and third harmonics. For example, one prior art system shown in FIG. 1 is configured for High Band (HB) Direction Finding (DF) wherein a Switch-Filter-Switch (SFS) is provided in order to select only the desired filter in the system so as to reduce the second and third harmonics of the fundamental frequency down to acceptable power levels. The arrangement as shown in FIG. 1 operates in a satisfactory manner in amplitude-only applications in which the DF legs are normally used.
Unfortunately, switches such as those used in FIG. 1, (i.e., the single-pole-quadruple-throw [1P4T]) switch and the single-pole-quintuple-throw [1P5T] switch) are typically reflective. Because of the reflective nature of such switches, a non-reflective output impedance match can only be obtained over the selected filter's frequency range. For example, if the filter that passes signals from about 2.0-3.5 GHz is selected by one of the switches, the output impedance match is poor in frequency ranges for the remaining three filters. Furthermore, when a different filter band is selected, such as the 6.0-10.4 GHz filter shown in FIG. 1, the output impedance of the 2-18 GHz system will change. In a multi-channel combining network, the changing output impedance will result in phase and amplitude errors in the signal each time the switch (e.g., the 1P5T switch of FIG. 1) changes which unfortunately results in errors in the DF capability of a system.
As can be seen, there exists a need in the art for a switched multiplexer that is configured to provide a consistent output impedance so as to minimize phase shifting when different band pass filters are selected from among a bank of filters. Furthermore, there exists a need in the art for a switched multiplexer that has the capability to improve the HB DF Leg's harmonic performance. More specifically, there exists a need in the art for a switched multiplexer wherein switches included in the system do not add any undesirable harmonics that are not filtered.